"Deterrence through the balance of terror", combined with computerised surveillance and early warning systems, ensured that any attack would be followed immediately by counterattack.

But MAD failed to take into account one dangerous possibility - computer error.

On the night of 26 September 1983, Stanislav Petrov, a young army software engineer, was on duty at a surveillance centre near Moscow.

Stanislav Petrov today

"Suddenly the screen in front of me turned bright red," said Petrov. "An alarm went off. It was piercing, loud enough to raise a dead man from his grave."

"The computer showed that the Americans had launched a nuclear strike against us."

A second alarm followed, then a third. "I knew the system could be at fault, so I reported this as a false alarm."

'A big risk'

A Soviet-era surveillance center

Petrov's orders were to pass the information up the chain of command, to the then General Secretary, Yuri Andropov. Within minutes, a massive nuclear counterattack would be launched.

Petrov's decision to disobey procedure was intuitive; "The thought crossed my mind that maybe someone had really launched a strike against us. That made it even harder to lift the receiver and say it was just a false alarm."

"I understood that I was taking a big risk."
When the computer error was reported, the army began a massive internal inquiry. But instead of being commended for his courage and quick thinking, Petrov was blamed.

Once a promising, twice-decorated young officer, Petrov took early retirement from the army and later suffered a nervous breakdown.

"I was made the scapegoat. That was our system, the old Soviet system, in the old Soviet army."